Simplified Preservation of Equal Paths Spectroscopy

Inspired by the recently proposed Transverse mixing Optimal control Pulses (TROP) approach for improving signal in multi-dimensional magic-angle spinning (MAS) NMR experiments, we present simplified preservation of equal paths spectroscopy (SPEPS). It transfers both transverse components of magnetization that occur during indirect evolutions, theoretically enabling a √2 improvement in sensitivity for each such dimension. We compare SPEPS transfer with TROP and cross polarization (CP) using membrane protein and fibril samples at MAS of 55 kHz and 100 kHz. In 3D (H)CANH spectra, SPEPS outperformed TROP and CP by factors of, on average, 1.16 and 1.69, respectively for the membrane protein, but only marginal improvement of 1.09 was observed for the fibril. These differences are discussed making note of the longer transfer time used for CP, 14 ms, as compared with 2.9 and 3.6 ms for SPEPS and TROP, respectively. Using SPEPS for two transfers in the 3D (H)CANCO experiment resulted in even larger benefit in signal intensity, with an average improvement of 1.82 as compared with CP. This results in a multifold time savings, in particular considering the weaker peaks that are observed to benefit the most from SPEPS.